How can I animate multiple objects to be highlighted in a scene?
You could duplicate all the objects (perhaps scale them up a percent, and maybe put them all in a separate collection), and give the all duplicates the same material with Make Links (Ctrl + L) &gt; Material. Then do something like make this material have a Mix Shader, with a Transparent BSDF and red Diffuse BSDF plugged into it. Then Animate the Fac value of the Mix Shader.

EDIT:
You could do the work but copy-paste the extra nodes and copy-paste a driver in the Mix Shader to decrease the amount of manual work.
Add an empty, add a custom property (Object Properties &gt; Custom Properties), give it a nice name (optional), right click the value field and choose Copy Data Path.

Create a node setup with a Mix Shader as an alternative output for a material. Give it a nice frame (optional). Right click the Fac value and choose Add Driver.

Edit the driver right away when it pop up, or with the Mix Shader selected go to the Drivers window. Type: Scripted Expression, Expression: &quot;var&quot;, for Prop target the empty object, and hoover over Path: with the mouse and press Ctrl+V to paste the custom property data path.

Sadly when you copy-paste the highlight nodes to another material the driver  isn't copied with it, but you can easily copy paste a driver by right clicking the field where the driver is applied. Connect it to the other nodes, and no need to delete anything; the last selected Material Output becomes the active one. (Once i had the nodes and driver set up, it took me only a few seconds to paste an connect them in to each material.)

Since all drivers share the data path they are all driven by the same custom property. And it can be animated.